A wide range of 2-dimensional (2-D) atomic crystals exist in nature. The simplest is graphene (an atomic-scale 2-D honeycomb lattice of carbon atoms), followed by Boron Nitride (BN). However, hundreds more exist including transition metal dichalcogenides (TMDs) such as Molybdenum disulphide (MoS2), Niobium diselenide (NbSe2), Vanadium telluride (VTe2), transmission metal oxides such as Manganese dioxide (MnO2) and other layered compounds such as Antimony telluride (Sb2Te3), Bismuth telluride (Bi2Te3). Depending on the exact atomic arrangement, these crystals can be metals, insulators or semiconductors.
Layered materials, come in many varieties with one family having the formula MXn (where M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mn, Mo, W, Tc, Re, Ni, Pd, Pt, Fe, Ru; X=O, S, Se, Te; and 1≦n≦3). A common group are the transition metal dichalcogenides (TMDs) which consist of hexagonal layers of metal atoms sandwiched between two layers of chalcogen atoms. While the bonding within these tri-layer sheets is covalent, adjacent sheets within a TMD crystal are weakly bound by van der Waals interactions. Depending on the co-ordination and oxidation state of the metal atoms, TMDs can be metallic or semiconducting. For example, Tungsten disulphide (WS2) is a semiconductor while Tantalum disulphide (TaS2) and Platinum telluride (PtTe2) are metals. This versatility makes them potentially useful in many areas of electronics.
However, like graphene, many believe that TMDs must be produced as flakes dispersed in liquids to facilitate processability and prevent re-aggregation. TMDs can be exfoliated by ion intercalation, as described in EP 0382339 and U.S. Pat. No. 4,822,590. However, this method is time consuming, extremely sensitive to the environment and incompatible with the majority of solvents and so is unsuitable for most applications. Furthermore, removal of the ions results in re-aggregation of the layers (R. Bissessur, J. Heising, W. Hirpo, M. Kanatzidis, Chemistry of Materials 1996, 8, 318).
Recently, it has been shown by the Applicants that graphite can be exfoliated to give graphene by sonication in certain solvents and that TMDs can be exfoliated in the same manner [References 1-8 (PCT Publication No. WO 2012/0287 being reference 8)]. This process gives dispersions of nanosheets stabilised in suitable solvents. It is possible to produce at least 1 litre (at a time) of graphene dispersed in solvents such as NMP at a concentration of at least 1 mg/ml i.e. >1 g of exfoliated graphene, using these methods. This means that 1 kg of graphene would require up to 1 m3 of solvent. For any company making graphene, this would make shipping difficult and costly, unless the solvent was removed which is possible but not trivial.
However, there are a number of problems with the procedures outlined in References 1-8. The exfoliation is carried out in special solvents such as n-methyl pyrrolidone (NMP), cyclohexylpyrrolidone, di-methyl formamide etc. These solvents must have surface tension in the vicinity of 40 mJ/m2 to match the surface energy (related to surface tension) of graphene. The reasoning behind the surface tension value is the energy cost of exfoliation (per volume of dispersion) of graphene which can be expressed as:
                                          Δ            ⁢                                                  ⁢                          H              Mix                                V                ≈                              2                          T              NS                                ⁢                                    (                                                                    E                                          S                      ,                      S                                                                      -                                                      E                                          S                      ,                      G                                                                                  )                        2                    ⁢                      ϕ            G                                              (        1        )            where TNS is the thickness of the graphene sheet, Es,s is the surface energy of the solvent, Es,G is the surface energy of the graphene and φG is the volume fraction (proportional to concentration) of the graphene. The surface tension requirement is to allow the graphene or other TMDs to be stabilised against aggregation and means a limited number of good solvents exist. Typically solvents with surface tensions close to 40 mJ/m2 are required to exfoliate most layered compounds (References 9-10). These solvents can also be expensive, difficult to remove due to high boiling points, and/or are toxic to the environment.
To remove the solvents after exfoliation, the mixture must be filtered through nanoporous membranes, which is very slow. Alternatively, a non-solvent or salt can be added to the mixture to destabilise the graphene, which sediments out and then can be collected. However, the solvent then has to be recycled which is slow and expensive.
Furthermore, Reference 8 (WO 2012/028724) describes that exfoliation is only possible in a water-surfactant mixture. It is believed that the surfactant is necessary for exfoliation and stabilisation of the exfoliated nanosheets. It is also believed that exfoliation and stabilisation cannot happen independently. While the document shows that vacuum filtration can be used to remove the solvent, this is only required because of the presence of the surfactant necessary to stabilise the exfoliated graphene. However, this process is slow and hard to scale.
EP 0382339 A1 does not describe a process of simply exfoliating a TMD in a simple solution. First of all, lithium ions must be intercalated between the layers of the TMD MoS2. This is a very slow process that cannot be achieved in ambient conditions. It is not scalable. Only after the Li has been intercalated can the exfoliation in water begin. Although evaporation can be used to remove the solvent, this is a slow and/or energy intensive process.
U.S. Pat. No. 4,822,590 A does not describe a process that can simply be described as exfoliation in a simple solution. As with EP 0382339 A1 above, lithium ions must first be intercalated between the layers of the MoS2. This is a very slow process that cannot be achieved in ambient conditions and it is not scalable. It is only after the lithium ions have been intercalated can the exfoliation begin. In the method of U.S. Pat. No. 4,822,590 A, centrifugation was used to remove the solvent. However, this is also a slow process of limited scalability.
There is therefore a need to provide two-dimensional atomic crystals by a suitable method or process to overcome at least one of the above-mentioned problems.